A pn junction type light-emitting diode (LED) has heretofore been known as a compound semiconductor light-emitting device (see, for example, “General Theory of Semiconductor Devices” by Iwao Teramoto, First Edition, Baifukan Publishing Co., Mar. 30, 1995, Chapter 7). For example, there has been known a GaP LED utilizing, as a light-emitting layer, a GaP layer obtained by epitaxially growing a single crystal of electrically conducting gallium phosphide (GaP) on a substrate (see the above “General Theory of Semiconductor Devices” by Iwao Teramoto, First Edition, Baifukan Publishing Co., Mar. 30, 1995, Chapter 7). There have further been known red band and orange yellowish to green band LEDs using a gallium aluminum arsenide mixed crystal (AlXGaYAs: 0≦X, Y≦1 and X+Y=1) and a gallium indium aluminum phosphide mixed crystal (AlXGaYInZP: 0≦X, Y, Z≦1 and X +Y+Z=1) as a light-emitting layers (see the above “General Theory of Semiconductor Devices” by Iwao Teramoto, First Edition, Baifukan Publishing Co., Mar. 30, 1995, Chapter 7). There has further been known a near ultraviolet band, blue band or green band short-wavelength LED using, as a light-emitting layer, a III-Group nitride semiconductor layer such as indium gallium nitride (GaαInβN: 0≦α, β≦1, α+β=1) (see, for example, Japanese Examined Patent Publication (Kokoku) No. 55-3834).
In the above AlXGaYInZP LED, a single crystal of electrically conducting p-type or n-type gallium arsenide (GaAs) is used as a substrate, and an electrically conducting n-type or p-type light-emitting layer is formed thereon. In the blue LED, a single crystal of electrically insulating sapphire (α-Al2O3 single crystal) is used as a substrate (see the above Japanese Examined Patent Publication (Kokoku) No. 55-3834). The short-wavelength LED further utilizes a cubic (3C crystalline) or hexagonal (4H or 6H crystalline) silicon carbide (SiC) as a substrate.
Usually, the following method is used to separate a wafer forming a plurality of semiconductor multilayer structure having light-emitting layer etc. on the wafer into the individual compound semiconductor light-emitting devices. (1) First, a semiconductor multilayer structure for LED is formed by forming an epitaxially grown layer that is necessary for constituting the LED on the surface of the wafer that serves as the substrate. (2) Next, negative electrodes and positive electrodes are arranged. (3) At the same time, linear stripe-like recessed grooves called separation zones are formed on the side of the front surface where the devices are formed, so as to be utilized for separation into individual devices, and scribe lines or cutting lines are formed along the separation zones. The cutting lines may be formed prior to forming the negative electrodes and the positive electrodes. At a moment when the scribe lines or the cutting lines are provided, however, the compound semiconductor light-emitting devices are still on the surface of the wafer that becomes the substrates and are arranged regularly and continuously. (4) To cut the wafer into individual devices, a cutting tool such as a diamond grinding blade that is revolving is moved linearly and horizontally in the recessed groove in the separation zone to cut and remove the substance forming the region where the wafer is to be divided. (5) Next, a mechanical pressure is applied from an external side onto the recessed groove that is further deepened due to the cutting. The wafer is cut by utilizing the mechanically weakened deep grooves and is divided into individual devices. According to another method, the wafer is divided into individual devices by utilizing the cleaving property of the substrate crystals or of the epitaxially grown layer by imparting continuous or intermittent mechanical shock to the scribe lines instead of effecting the cutting to deepen the recessed grooves. In dividing the wafer into individual devices by utilizing the cleaving property, it is a widely accepted practice to form the scribe lines in a direction in which the substrate crystal or the grown layer cleaves, e.g., in parallel with the <110> direction in the case of zincblende crystals such as GaAs having a strong ionic bond.
A technology for increasing the surface areas of the side surfaces of an LED having, as side surfaces, cut surfaces or cleaved surfaces exposed along the scribe lines, is advantageous for increasing the efficiency for outwardly transmitting the emitted light. If this technology is utilized for a compound semiconductor light-emitting device that features excellent efficiency for outwardly transmitting the emitted light, it is allowed to constitute a lamp featuring a higher brightness yet consuming the same amount of electric power. In a nearly square LED having a side of a length of 200 μm to 450 μm, however, the separation zone in which the cutting line or the scribe line is provided has a width (transverse width) of, usually, from 20 μm to 50 μm at the greatest. The separation zone forms a region that does not contribute to emitting light. If this region is wide, the region that contributes to forming the light-emitting device decreases on the substrate wafer and the cost of production increases.
As described above, the width (transverse width) of the separation zone is as narrow as the blade tip of the cutting tool. Therefore, the blade tip of the cutting tool is not allowed to move in a line shifted longitudinally and transversely. If forcibly moved longitudinally and transversely, the blade tip of the cutting tool deviates from the recessed groove of the line and it becomes probable that the light-emitting layer and the electrode are cut or damaged causing the devices to become defective. It is therefore difficult to obtain side surfaces of large surface areas having a zig-zag plane in cross section.
The same also holds even when the wafer is divided into individual devices relying upon the cleavage by the scribing method. For example, to form side surfaces having large surface areas, the mechanical damage for triggering the cleavage must be imparted in an undulating manner or a zig-zag manner. Despite the mechanical damage being imparted in an undulating manner, however, there still remains the inconvenience that the wafer is linearly divided in the cleaving direction naturally in compliance with the cleaving property without following the direction of imparting the damage. Namely, according to the prior art, the resultant side surfaces are nearly planes irrespective of whether cutting or cleavage is used. In particular, the side surfaces obtained by the cleaving method are flat surfaces having smoothness favorable for being utilized as laser beam resonance surfaces. Therefore, it is difficult to produce an LED, having side surfaces of large surface area, which improves the efficiency for outwardly transmitting the emitted light.
In Japanese Laid-Open Patent Application (kokai) No. 2003-110136, which increases the efficiency for outwardly transmitting the emitted light by means of making the device side surface rugged, the undulating ruggedness is formed in the side surface of compound semiconductor layers formed on the sapphire substrate by etching. However, because the compound semiconductor layers formed on the sapphire substrate is very thin, the resultant increased amount of surface area is very small.